1. Field of the Invention
The present invention relates generally to contact lenses, more particularly to a contact lens of novel structure which during wear on a cornea of an eye is capable of minimizing noticeable change upon decentering of an optical axis of an optical zone of the contact lens with respect to an optical axis of the eye, and to techniques related to such a contact lens.
2. Description of the Related Art
Conventionally, soft contact lenses and hard contact lenses (hereinafter referred to collectively as “contact lenses”) such as those disclosed in Japanese Patent Nos. 2,859,092 and 2,913,191, were generally designed with only a lens unit taken into consideration, like in optical lenses for use in other optical devices, such as cameras. Namely, the contact lenses were generally designed by shaping the lens surface according to a ray tracing method so as to achieve a desired refractive power through space. However, a contact lens is worn on a cornea of an eye, and it is therefore no longer possible to ignore factors such as the optical system of the eye per se, the lens effect resulting from a tear fluid layer formed between the contact lens and the cornea during wear (tear-fluid lens effect) and the like. For this reason, the limits of the above-described optical design based on only the contact lens unit have been pointed out, recently. A method for addressing this problem is proposed in Japanese Patent No. 3,022,640, in which a mathematical model of a system composed of a human eye and a preliminary lens is created, and the optical system of the eye is considered using this mathematical model, to thereby minimize aberration so as to give a lens surface shape with improved optical characteristics.
However, regardless of whether the optical system of the eye is considered or not, the conventional contact lens design methods disclosed in Japanese Patent Nos. 2,859,092, 2,913,191 and 3,022,640 are simply desired to optimizing optical characteristics on the optical axis of the contact lens. In this respect, an extensive research conducted by the inventors, as well as numerous clinical trials and studies based thereon have revealed that the presumption in lens design that emphasizes optical characteristics on the optical axis of the contact lens is in itself highly problematic, and that contact lenses deemed optimal under conventional contact lens design methods may practically fail to provide optimal correction of vision of lens wearers.
The principal reasons way optimizing optical characteristics on the optical axis of the contact lens (i.e., an optical center axis of an optical zone of the contact lens, hereinafter referred to as a “lens optical axis”) does not necessarily optimally correct vision during contact lens wear, are given in (1)–(3) hereinbelow:    (1) Firstly, a contact lens worn on a cornea is normally tends to come to rest at the ear-side lower portion of the cornea due to effects of gravity, pulling pressure between a tear fluid and the lens, a nonuniform surface curvature of the cornea, and the like. As a result, the contact lens is worn on the cornea in a position in which the lens optical axis of the contact lens, which is set to a geometric center axis of the contact lens, is offset from an optical axis passing through the pupil (hereinafter referred to as a “pupil centerline”).    (2) Secondly, a contact lens is designed to move appropriately over the cornea during blinking for the purpose of tear fluid exchange and the like, and accordingly the lens optical axis is not maintained in alignment with the pupil centerline during such frequent movements.    (3) Thirdly, human vision is affected significantly not only by acuity at the fixation point on the pupil centerline, but also by acuity at peripheral locations further away from the pupil centerline (hereinafter referred to as a “peripheral vision”). Therefore, corneal corrective surgery performed with the object of eliminating image aberration on the pupil centerline, for example, may possibly causes aberration further away from the pupil centerline, which conversely increases due to lack of aberration on the pupil centerline, resulting in adverse effects such as poor vision and headache.
In recent years, on the other hand, there has also been proposed to give an a spherical shape to back surfaces of contact lenses, for the purpose of having the shape of the back surface of the contact lens conform to the shape of the cornea, as well as providing appropriate exchange of tear fluid so as to improve contact lens wear comfort. Extensive studies of aspheric lenses of this kind by the present inventors has revealed that when a lens front surface is designed using conventional lens design methods that optimize optical characteristics on the lens optical axis as described above, aberration in a visual field further away from the lens optical axis tends to increase more. Accordingly, where contact lenses typically come to rest at positions with the lens optical axes offset from the pupil centerlines as described above, lenses that have been designed optimally according to conventional lens design methods tend to produce problems such as significantly impaired vision in the course of actual wear.